This invention concerns an AC/DC converter provided with a control system which has the facility to control the converter""s operation with respect to generation of harmonics in an AC power system to which it is connected.
The converter may be a 6 pulse or 12 pulse type comprising six or twelve thyristors or groups of series-connected thyristors connected via a transformer to 3-phase AC busbars connected to an AC source via a load.
Normally at least one harmonic filter is provided in shunt to the busbars primarily to filter out characteristic harmonic currents produced by the converter. For example, for a 12-pulse converter, the harmonic currents may be of the order 11, 13, 23 and 25, etc.
In normal operation, first assuming a perfectly balanced 3-phase AC system, and also perfect balance of internal components in the converter system (such as transformer leakage inductances), the AC side harmonics generated by a 12-pulse converter, for example, will be at multiples of AC system frequency of 11, 13, 23, 25, . . . The DC side harmonics are of orders 12, 24, 36 . . .
In practice such perfect balance cannot be guaranteed, and for example the AC busbar voltages may contain a negative sequence fundamental component, due mostly to unbalance of AC line impedances and unequal loads between the three phases in the AC system. It can be shown that on the DC side of the converter this will cause a 2nd harmonic voltage component to appear. This causes a 2nd harmonic current on the DC side, which in turn modifies the AC currents drawn by the converter from the AC system so that they contain 3rd harmonic components. The effect of the latter is to cause 3rd harmonic voltages both locally on the converter busbars, and also in remote parts of the AC system. These voltages may have an amplitude of up to typically 3% of rated converter current, and may cause unacceptable interference to other equipment in the AC system.
A well known method to reduce such interference is to provide passive filters tuned to the 3rd harmonic on the AC busbars. These can however be of substantial cost, particularly because of the relatively low frequency (3rd harmonic) to be filtered.
A general object of the invention is to provide a thyristor-type AC/DC converter with an auxiliary control system and method for the attenuation or suppression to zero or low values of a non-characteristic component of voltage or current in an AC system connected to the AC/DC converter, in the presence of an unbalance, without use of filters corresponding to that noncharacteristic component.
One particular object of the invention is to provide such an AC/DC converter with an auxiliary control system, and a method of controlling such a system, which can reduce 3rd harmonic currents and voltages in the AC system to zero or low values, in the presence of unbalance, without use of 3rd harmonic filters. A further object is to provide such an auxiliary control means which may also be adapted to reduce other low order AC currents, for example harmonic orders 4, 5, 6 or 7, in the case of a twelve-pulse converter.
According to the invention there is provided a thyristor-type AC/DC converter for connection to an AC power system, the converter having a main control system for supplying firing pulses to a plurality of thyristors in the converter and an auxiliary control system connected to the main control system through a summing junction for modifying the timing of the firing pulses from the main control system, thereby to suppress a non-characteristic nth harmonic component of a fundamental frequency in the AC power system caused by the operation of the AC/DC converter, the auxiliary control system being connected to receive a three-phase input signal representative of the harmonic component and deliver to the summing junction an output signal comprising a modulating rth harmonic signal, the auxiliary control system comprising a two-axis integral AC servo control.
More particularly, the auxiliary control system preferably comprises:
a three-phase to two-phase conversion means for converting the input signal to two signals containing both the fundamental frequency and the nth harmonic component,
demodulating means for demodulating the two signals from the conversion means to produce two DC signals proportional respectively to direct axis and quadrature axis components of the nth harmonic component,
co-ordinate transform means for phase rotation of the two DC signals from the demodulating means by a phase angle xcex,
integration means for integration of the phase rotated signals from the signal co-ordinate transform means to produce two integrated DC signal components, and
modulating means for modulating and combining the two integrated DC signal components from the signal integration means to form the rth harmonic output signal from the auxiliary control system.
The AC/DC converter may comprise a 6-pulse or 12-pulse converter.
In one preferred embodiment, the non-characteristic nth harmonic is the 3rd harmonic and the modulating rth harmonic is the 2nd harmonic.
The auxiliary control system may have gain and phase angle settings which are manually adjustable.
Alternatively, the auxiliary control system may have gain and phase angle settings which may be adjusted in dependence on measured quantities in the converter according to predetermined relationships. The measured quantities may comprise converter DC voltage and converter DC current.
The auxiliary control system may have gain and phase angle settings which are adjusted by a self-adaptive control system based on an automatic two-stage test method carried out with the auxiliary control system in an open-loop mode to modify the gain and phase settings to optimum values. If desired, the gain and phase angle settings of the auxiliary control system may be set to values substantially equal to the reciprocal of a measured complex value of total loop gain of a network comprising the main control system, AC/DC converter, AC system and at least part of the auxiliary control system. The test and modification of the gain and phase settings may be repeated on detection of substantial disturbances in operation of the AC/DC converter.
The auxiliary control system may have a phase angle setting which may be adjusted by a self-adaptive control system based on a single-stage test carried out with the auxiliary control system in a closed-loop control mode by a comparison of phases of the input and output signals of the auxiliary control system and subsequent automatic transfer of an angle derived from such comparison to become a working angle of the auxiliary control system. The phase comparison and angle transfer may be initiated at first switch-on of the auxiliary control system and/or on detection of substantial disturbances in operation of the AC/DC converter.